1. Field of the Invention
The present invention relates to an image forming apparatus which transfers a toner image formed on the photosensitive layer onto a recording medium, or an image forming apparatus which transfers toner images of multiple colors formed on the photosensitive layer onto a transfer target medium by superimposition to print a colored image. More particularly, the present invention relates to an image forming apparatus which is capable of preventing the dropout of image even if the impedance between the base cylinder which supports the photosensitive layer and the transfer device varies due to the long-term use or varies depending on the type of recording medium.
2. Description of the Related Art
The image forming apparatus based on electrophotography operates to transfer a toner image formed on the photosensitive layer onto a recording medium such as print paper (will be termed "print paper" hereinafter) based on the application of a static electric field. FIG. 13 shows in a sense of model the cross section of the place where the transfer of image is taking place. Specifically, the figure shows the cross section of the place where a photosensitive drum 10 which has a photosensitive layer 12 formed on a base cylinder 11 and a carrying belt 26 which bears and transports a sheet of print paper P are closest to each other (this place will be called "transfer zone" hereinafter).
On the rear side of the carrying belt 26 at the transfer zone, there is disposed a transfer brush 20 which charges the carrying belt 26 to produce an electric field. The transfer brush 20 has the application of a positive voltage relative to the base cylinder 11 by a transfer power source 21. In operation, when the transfer brush 20 is supplied with the positive voltage from the transfer power source 21, toner T which is negatively charged on the photosensitive layer 12 is attracted by the electric field existing between the photosensitive layer 12 and the print paper P and transferred to the print paper P.
In the case of a multi-color image forming apparatus, toner images of individual colors are formed on the photosensitive layer and transferred to the print paper one by one.
FIG. 14 shows the distribution of electric field generation in the place between the photosensitive layer 12 and the print paper P. The graph reveals that the magnitude of electric field has peaks at both edges of a toner image and is smaller in the central portion of the image. This electric field distribution is conceivably attributable to the effect of the lateral component of electric field resulting from different electrical conditions between an image portion with toner (will be called "toner image portion") and an image portion without toner (will be called "non-toner image portion"). Two major electrical conditions relevant to this situation are: (1) the surface potential of the photosensitive layer 12, and (2) the impedance between the base cylinder 11 and the transfer power source 21.
The following explains the impedance between the base cylinder 11 and the transfer power source 21. The impedance of the circuit of the section shown in FIG. 13 is treated in terms of the equivalent circuit as shown in FIG. 12. Specifically, the impedance of this section is equivalently a parallel connection of an impedance Z.sub.0 of the toner image portion and impedances Z.sub.1 of the two non-toner image portions.
The impedance Z.sub.0 of toner image portion is expressed to be a serial connection of the impedances Z.sub.C, Z.sub.T, Z.sub.P and Z.sub.B of the photosensitive layer 12, layer of toner T, print paper P, and carrying belt 26 (inclusive of the transfer brush 20), respectively, as follows. EQU Z.sub.0 =Z.sub.C +Z.sub.T +Z.sub.P +Z.sub.B ( 1)
The impedance Z.sub.1 of non-toner image portion is expressed similarly to the impedance Z.sub.0 of toner image portion, with the impedance Z.sub.T of toner layer being replaced with the impedance Z.sub.A of air gap as follows. EQU Z.sub.1 =Z.sub.C +Z.sub.A +Z.sub.P +Z.sub.B ( 2)
Accordingly, the difference between Z.sub.0 and Z.sub.1 resulting from their Z.sub.T and Z.sub.A (Z.sub.T &gt;Z.sub.A, thus Z.sub.0 &gt;Z.sub.1) is the major basis of the electric field distribution shown on the graph of FIG. 14.
It should be noted that the equivalent circuit of FIG. 12 and the above expressions (1) and (2) do not directly show themselves the effect of the lateral component of electric field. The electric field distribution shown in FIG. 14 is not solely attributable to the difference of the above-mentioned two conditions between the toner image portion and non-toner image portions, but the edge effect resulting from the difference also acts on the lateral component of electric field. On this account, for a wide toner image portion, a weaker electric field is liable to emerge immediately inside of the edge of toner image rather than the center of image, as shown on the graph of FIG. 16.
This uneven electric field distribution across the toner image results presumably in the occurrence of dropout in the toner image formed on the print paper P, because of insufficient transfer of toner in the central portion of toner image where the electric field strength can be weak relatively.
For coping with this matter, practical image forming apparatus are designed to set the transfer output such that toner is transferred sufficiently to the toner image center where the electric field strength is weak relatively. In addition, some image forming apparatus is designed to control the transfer output depending on such an environmental factor as the humidity thereby to cope with the variation in the impedance of the air gap and print paper, as described in JP-A-Hei-4-190381 for example. Some multi-color image forming apparatus is designed to control the transfer current separately for the photosensitive substance of each color, as describe in JP-A-Sho-63-228179.
However, the foregoing conventional image forming apparatus have the following problems.
Both the impedances Z.sub.0 and Z.sub.1 of the toner image portion and non-toner image portion include the impedance Z.sub.C of the photosensitive layer 12, as indicated by the expressions (1) and (2). If the thickness of photosensitive layer 12 decreases due to the long-term use, the impedance Z.sub.C becomes smaller, and Z.sub.C and Z.sub.A of the toner and air gap contribute increasingly to the values of Z.sub.0 and Z.sub.1. The value of ratio Z.sub.0 /Z.sub.1 (greater than one) increases progressively during the long-term use of the photosensitive layer 12, and the unevenness of the electric field distribution grows (refer to FIG. 15). Consequently, even though the transfer output is set appropriately for a new photosensitive layer 12, the dropout will be liable to occur during the long-term use.
Another problem is induced by the influence of the kind of print paper P, since the impedances Z.sub.0 and Z.sub.1 also include the impedance Z.sub.P, of print paper P. Specifically, print paper P having a smaller impedance is liable to cause the dropout similar to the case of the long-term use. Moreover, print paper P has its impedance affected by the environment (particularly, the humidity), and therefore it can have a smaller impedance.
Although these problems can conceivably be coped with based on the control of transfer output by the apparatus proposed in the above-mentioned publication, the unevenness of electric field distribution cannot be alleviated by the transfer output control which merely moves vertically the whole curve of electric field distribution of FIG. 15. Accordingly, increasing the transfer output will prevent the occurrence of dropout, while at the same time an excessive transfer output will incur such a side effect as reverse (recurrent) transfer at the edge section.
In the case of a multi-color image forming apparatus, the dropout occurring particularly in a black toner image severely damages the picture quality. The reason is that even a colored image is dominated by black portions for characters and thin lines, and it is printed by mainly using black toner and using little toner of other colors (cyanine, magenta and yellow) in general. The image forming apparatus of the above-mentioned JP-A-Sho-63-228179 does not consider the significance of black toner.